In general, as a spark plug in which a resistor is mounted, there is one having a structure with a cylindrical insulator body, a through-hole formed inside the insulator along its axial direction, a terminal metal piece made of metal inserted through one end of the through-hole to be fixed thereto, a center electrode made of metal inserted through the other end thereof and fixed, and a resistor disposed between the terminal metal piece and the center electrode in the through-hole (for example, see Japanese Patent Document JP-A-11-339925). The resistor is made of a mixture of glass powder and a conductive material such as carbon black powder or metal powder, however, the metal content is not that high. Accordingly, in many cases, a direct connection between the terminal metal piece made of metal and the center electrode is difficult. Therefore, generally, a configuration has been employed in which a conductive glass sealing layer made of a mixture from glass powder and a relatively large amount of metal powder is provided between them to enhance the joining force.
The spark plug having such a resistor is manufactured as follows.
First, after inserting and fixing the center electrode to the through-hole of the insulator, the conductive glass powder is charged. Thereafter, the raw powder having the resistor composition is charged, the conductive glass power is charged again, and finally, the terminal metal piece is press-fitted from the opposite side to the center electrode, thereby obtaining an assembled unit. Next, the assembled unit is placed in a heating furnace and heated to above a glass softening temperature, and the terminal metal piece is pushed in the axial direction of the terminal metal piece so as to compact each layer. As described above, the conductive glass sealing layer on the resistor and the center electrode side and the conductive glass sealing layer on the terminal metal piece side are obtained. In addition, the terminal metal piece and the center electrode are joined to the resistor with the respective conductive glass sealing layers interposed therebetween and fixed to the insulator (hereinafter, the process for forming the layers is referred to as a “layer formation process”).
The spark plug disclosed in Japanese Patent Document JP-A-11-339925 is provided with a metal layer made of a specific material at a surface region of a portion which comes in contact with the conductive glass sealing layer of the terminal metal piece, and by increasing the joining force between the terminal metal piece and the conductive glass sealing layer, problems such as deterioration of the joining state are prevented.
However, recently, in order to achieve an engine design with the ability to allow for high engine power outputs and high engine efficiencies by such means as the increasing of a valve diameter and the free processing a water jacket for more coolant, or in order to save space by reducing the engine itself, a reduction in diameter of the spark plug has been strongly required in terms of vehicle design. In order to realize this, a reduction in diameter of the insulator is inevitable.
However, in order to ensure voltage resistance performance and mechanical strength that are requirements of the insulator, a predetermined radial thickness is needed. Accordingly, the through-hole of the insulator, and in turn, the diameters of the resistor and the conductive glass sealing layer have to be reduced.
However, when the diameter of the through-hole of the insulator is reduced, the sufficient charging of the conductive glass powder is difficult. Moreover, there is a tendency that mechanical strength between the resistor and the conductive glass sealing layer, and particularly, of the joining portion of the resistor and the conductive glass sealing layer is insufficient. Therefore, in some cases, when vibration or an impact is exerted on the spark plug from the engine, exfoliation occurs at the joining surface of the resistor and the conductive glass sealing layer, which does not occur in an existing spark plug where a through-hole has a diameter of about 3.9 mm, so that there are problems with electrical connection.
The spark plug disclosed in Japanese Patent Document JP-A-11-339925 is provided with a metal layer made of a specific material at a surface region of the terminal metal piece and configured to prevent deterioration of a joining state or the separation of the terminal metal piece by increasing the joining force between the terminal metal piece and the conductive glass sealing layer. Therefore, it is associated with a different technique from that of the invention which has an object to reduce the diameter of the spark plug.
In order to solve the above-mentioned problems, it is an object of the invention to provide a spark plug with excellent vibration resistance performance and resistor load life-span characteristics and a reduced diameter which is achieved by strengthening adhesion between a resistor and a conductive glass sealing layer.